The present application relates to an optical drive device for performing recording and/or reproduction with respect to an optical disc recording medium and a focus servo control method thereof and, more particularly, to an optical drive device configured to irradiate a recording/reproducing light for recording (or reproducing) a mark and a servo light for performing servo control based on a position guide element formed in the optical disc recording medium through a common objective lens.
As an optical recording medium for performing recording/reproduction of a signal by light irradiation, for example, a so-called optical disc such as a Compact Disc (CD), Digital Versatile Disc (DVD) or Blu-ray Disc (BD) (registered trademark) have come into wide use.
With respect to an optical recording medium which is a next-generation optical recording medium widely used in the present state of the CD, the DVD, the BD and the like, first, the present applicant proposes a so-called bulk recording type optical recording medium described in Japanese Unexamined Patent Application Publication No. 2008-135144 or 2008-176902.
Japanese Unexamined Patent Application Publication No. 2009-9635 is also an example of related art.
Here, bulk recording indicates, for example, a technology of realizing a large amount of recording capacity by irradiating a laser beam to an optical recording medium (bulk type recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 by sequentially changing a focal point position so as to perform multi-layer recording in the bulk layer 102, as shown in FIG. 15.
In such bulk recording, Japanese Unexamined Patent Application Publication No. 2008-135144 discloses recording technology which is called a so-called micro hologram method.
The micro hologram method is classified broadly into a positive type micro hologram method and a negative type micro hologram method, as shown in FIGS. 16A and 16B.
In the micro hologram method, a so-called hologram recording material is used as a recording material of the bulk layer 102. As the hologram recording material, for example, a photopolymerizable photopolymer is widely used.
As shown in FIG. 16A, the positive type micro hologram method is a method of focusing two opposing light fluxes (light flux A and light flux B) at the same position so as to form a minute interference fringe (hologram) and using the minute interference fringe as a recording mark.
The negative type micro hologram method shown in FIG. 16B is a method of erasing an interference fringe formed in advance by laser beam irradiation and using the erased portion as a recording mark, in opposition to the positive type micro hologram method.
FIGS. 17A and 17B are diagrams illustrating the negative type micro hologram method.
In the negative type micro hologram method, before performing a recording operation, as shown in FIG. 17A, an initialization process for forming an interference fringe in the bulk layer 102 is performed in advance. In detail, as shown in the drawing, light fluxes C and D by parallel lights are oppositely irradiated so as to form such an interference fringe in the overall bulk layer 102.
After the interference fringe is formed in advance by the initialization process, as shown in FIG. 17B, information recording is performed by forming an erasing mark. In detail, by irradiating a laser beam according to recording information in a state of focusing on an arbitrary layer position, information recording by the erasing mark is performed.
The present applicant proposes, for example, a recording method of forming a void (hole) disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902 as a recording mark, as a bulk recording method different from the micro hologram method.
The void recording method is, for example, a method of irradiating a laser beam to the bulk layer 102 formed of a recording material such as a photopolymerizable photopolymer with relatively high power so as to record a hole (void) in the bulk layer 102. As described in Japanese Unexamined Patent Application Publication No. 2008-176902, the formed hole portion has a refractive index different from that of the other portion of the bulk layer 102 and thus the light reflection ratio of the boundary portion thereof is increased. Accordingly, the hole portion functions as a recording mark and thus information recording by formation of a hole mark is realized.
In such a void recording method, since the hologram is not formed, recording is completed by light irradiation from one side. That is, as in the positive type micro hologram method, it is not necessary to focus two light fluxes at the same position so as to form the recording mark.
In addition, in comparison with the negative type micro hologram method, it is an advantage that the initialization process is not performed.
In Japanese Unexamined Patent Application Publication No. 2008-176902, although an example of irradiating a pre-cure light before recording at the time of performing void recording is described, void recording is possible even when the irradiation of the pre-cure light is omitted.
However, even in the bulk recording type (simply also referred to as bulk type) optical disc recording medium in which the above various recording methods are proposed, the recording layer (bulk layer) of the bulk type optical disc recording medium does not have an explicit multi-layer structure in the sense that, for example, a plurality of reflection films is formed. That is, in the bulk layer 102, a reflection film and a guide groove for every recording layer including a general multi-layer disc are not provided.
Accordingly, in the structure of the bulk type recording medium 100 as it is shown in FIG. 15, focus servo and tracking servo may not be performed during recording in which the mark is not formed.
Accordingly, practically, in the bulk type recording medium 100, a reflection surface (reference surface) is provided which becomes a reference having guide grooves shown in FIG. 18.
In detail, the guide grooves (position guide element) such as pits or grooves are formed in a lower surface side of the cover layer 101 and a selective reflection film 103 is formed on the guide grooves. The bulk layer 102 is laminated on the lower layer side of the cover layer 101, on which the selective reflection film 103 is formed, with an adhesive material interposed therebetween as an intermediate layer 104 in the drawing, such as a UV curing resin.
Here, by the formation of the guide grooves such as the pits or the grooves, for example, recording of absolute position information such as radius position information or rotation angle information is performed. In the following description, a surface (in this case, a surface on which the selective reflection film 103 is formed) in which such guide grooves are formed and the absolute position information is recorded is referred to as a “reference surface Ref”.
After such a medium structure is formed, as shown in FIG. 19, separately from a laser beam (hereinafter, referred to as a recording/reproducing laser beam or simply a recording/reproducing light) for recording (or reproducing) a mark, a servo laser beam (simply referred to as a servo light) as a laser beam for position control is irradiated to the bulk type recording medium 100.
As shown, the recording/reproducing laser beam and the servo laser beam are irradiated to the bulk type recording medium 100 through a common objective lens.
At this time, if the servo laser beam reaches the bulk layer 102, the mark recording in the bulk layer 102 may be adversely affected. Accordingly, in the bulk recording method of the related art, the laser beam having a wavelength range different from that of the recording/reproducing laser beam is used as the servo laser beam, and the selective reflection film 103 having wavelength selectivity, which reflects the servo laser beam and transmits the recording/reproducing laser beam is provided as the reflection film formed on the reference surface Ref.
On the above assumption, the operation at the time of mark recording for the bulk type recording medium 100 will be described with reference to FIG. 19.
First, when multi-layer recording is performed with respect to the bulk layer 102 in which the guide grooves and the reflection film are not formed, the layer position for recording the mark in a depth direction in the bulk layer 102 is set in advance. In the drawing, the case where a total of 5 information recording layers (mark forming layers) L including a first information recording layer L1 to a fifth information recording layer L5 is set as a layer position (mark forming layer; also referred to as an information recording layer) for forming the mark in the bulk layer 102 is shown. As shown, the layer position of the first information recording layer L1 is set to a position separated by a first offset of-L1 in a focus direction (depth direction) from the selective reflection film 103 (reference surface) in which the guide grooves are formed. The layer position of the second information recording layer L2, the layer position of the third information recording layer L3, the layer position of the fourth information recording layer L4 and the layer position of the fifth information recording layer L5 are set to positions separated from the selective reflection film 103 by a second offset of-L2, a third offset of-L3, a fourth offset of-L4 and a fifth offset of-L5, respectively.
During recording in which the mark is not yet formed, focus servo and tracking servo are not performed based on the reflected light of the recording/reproducing laser beam with respect to the layer positions in the bulk layer 102 as a target. Accordingly, the focus servo control and the tracking servo control of the objective lens during recording are performed so as to enable the spot position of the servo laser beam to follow the guide grooves on the reference surface Ref (selective reflection film 103) based on the reflected light of the servo laser beam.
It is necessary for the recording/reproducing laser beam to reach the bulk layer 102 formed on the lower layer side of the selective reflection film 103, for mark recording. To this end, in an optical system of this case, a focus mechanism for the recording/reproducing light is provided, which independently adjusts a focusing position of the recording/reproducing laser beam separately from the focus mechanism of the objective lens.
Here, a configuration example of an optical system for performing recording and reproduction of the bulk type recording medium 100 including the mechanism for independently adjusting the focusing position of the recording/reproducing laser beam is shown in FIG. 20.
In FIG. 20, as shown, the objective lens shown in FIG. 19 may be displaced by the biaxial actuator in a radius direction (tracking direction) of the bulk type recording medium 100 and a direction (focus direction) abutting on or separated from the bulk type recording medium 100.
In FIG. 20, the mechanism for independently adjusting the focusing position of the recording/reproducing laser beam corresponds to a focus mechanism expander shown in the drawing. In detail, the focus mechanism as the expander includes a fixed lens and a movable lens which is held so as displaceably in a direction parallel to an optical axis of the recording/reproducing laser beam by a lens driving unit. By driving the movable lens by the lens driving unit, the collimation of the recording/reproducing laser beam incident to the objective lens in the drawing is changed and thus the focusing position of the recording/reproducing laser beam is adjusted independently of the servo laser beam.
As described above, since the recording/reproducing laser beam and the servo laser beam have different wavelength ranges, in correspondence therewith, in the optical system of this case, the reflected lights from the bulk type recording medium 100 of the recording/reproducing laser beam and the servo laser beam are separated to the respective systems (that is, the respective reflected lights are independently detected) by a dichroic prism as in the drawing.
In a forward light, the dichroic prism has a function for synthesizing the recording/reproducing laser beam and the servo laser beam on the same optical axis so as to enable the synthesized beam to enter the objective lens. In detail, in this case, as shown, the recording/reproducing laser beam is reflected from a mirror through the expander and then is reflected from a selective reflection surface of the dichroic prism so as to enter the objective lens. In contrast, the servo laser beam transmits the selective reflection surfaces of the dichroic prism so as to enter the objective lens.
FIG. 21 is a diagram illustrating servo control at the time of the reproduction of the bulk type recording medium 100.
During reproduction of the bulk type recording medium 100 in which the mark recording is already performed, it is not necessary to control the position of the objective lens based on the reflected light of the servo laser beam as during recording. That is, during reproduction, focus servo control and tracking servo control of the objective lens may be performed to the mark string formed in the information recording layer L to be reproduced as a target based on the reflected light of the recording/reproducing laser beam.
As described above, in the bulk recording method, the recording/reproducing laser beam for recording/reproducing a mark and the servo light as the position control beam are irradiated to the bulk type recording medium 100 through the common objective lens (synthesized on the same optical axis), the focus servo control and the tracking servo control of the objective lens are performed during recording such that the servo laser beam follows the guide grooves of the reference surface Ref, and the focal point position of the recording/reproducing laser beam is separately adjusted by the focus mechanism for the recording/reproducing light such that the mark is recorded at the necessary position (in the depth direction and the tracking direction) in the bulk layer 102 even when the guide grooves are not formed in the bulk layer 102.
In addition, during reproduction, by performing the focus servo control and the tracking servo control of the objective lens based on the reflected light of the recording/reproducing laser beam such that the focal point position of the recording/reproducing laser beam follows the mark string recorded in advance, it is possible to reproduce the mark recorded in the bulk layer 102.
When considering only the reproduction of the mark, in the servo control during reproduction, it is necessary to perform only servo control based on the recording/reproducing laser beam as described above. However, in practice, during the reproduction of the mark, the servo control for the reference surface Ref of the servo laser beam may be requested, for the reason that the reading of the absolute position information recorded on the reference surface Ref is performed.
At this time, in the case where the mark string formed in a certain information recording layer L is reproduced, if the focus servo control of the objective lens based on the reflected light of the recording/reproducing laser beam in a state in which the focus mechanism of the recording/reproducing laser beam shown in FIG. 20 is driven by the same amount as during recording, the focal point position of the servo laser beam ideally matches on the reference surface Ref (the driving state in the focus direction of the objective lens is ideally the same as during recording).
However, in practice, during recording, even the servo of the servo laser beam for the reference surface Ref as a target is performed as described above, the focusing position (that is, the mark recording position) of the recording/reproducing laser beam may not be maintained on the information recording layer L selected to be recorded. In other words, in practice, it is difficult to maintain the gap between the reference surface Ref and the mark recording position in the depth direction by the offset of-L corresponding to the information recording layer L as a target.
In detail, FIG. 22 illustrates the relationship between the reference surface Ref and the recorded mark string in practice.
That is, as shown in FIG. 22, the recorded mark string is not parallel to the reference surface Ref.
As described above, during reproduction, when only the servo control of the objective lens based on the recording/reproducing laser beam is performed, the focal point position of the servo laser beam may not match to the reference surface Ref.
In addition, the phenomenon shown in FIG. 22 occurs because the adjustment of the focusing position of the recording/reproducing laser beam during recording is performed by changing the collimation of the recording/reproducing laser beam incident to the expander shown in FIG. 20, that is, the objective lens.
The expander may adjust the focusing position of the recording/reproducing laser beam at a position different from the focusing position of the servo laser beam incident to the objective lens as a parallel light by enabling the recording/reproducing laser beam to enter the objective lens as a non-parallel light. However, by enabling the recording/reproducing laser beam to enter the objective lens as the non-parallel light, if the objective lens is driven in the focus direction by follow-up to surface wobbling or the like of the bulk type recording medium 100, the incident diameter of the recording/reproducing laser beam to the objective lens is changed and, as a result, the focusing position of the recording/reproducing laser beam is changed from the original position. Accordingly, the gap between the recorded mark string (the focusing position of the recording/reproducing laser beam) and the reference surface Ref (the focusing position of the servo laser beam) is not uniform and the non-parallel relationship shown in FIG. 22 occurs.
In order to solve such a problem, in the related art, as shown in FIG. 23, a configuration is proposed in which a focus mechanism is separately provided on the servo laser beam side.
In FIG. 23, a recording/reproducing laser 111, a collimation lens 112, a beam splitter 113, a focus mechanism 114 for a recording/reproducing light, a lens driving unit 115, a mirror 116, a focusing lens 120 and a photodetector 121 in the drawing configure an optical system of a recording/reproducing laser beam. In addition, a servo laser 122, a collimation lens 123, a beam splitter 124, a focus mechanism 125 for a servo light, a lens driving unit 126, a focusing lens 127 and a photodetector 128 in the drawing configure an optical system of a servo laser beam.
In addition, a dichroic prism 117, an objective lens 118 and a biaxial actuator 119 configure a common optical system of the recording/reproducing laser beam and the servo laser beam, as can be understood from the description with reference to FIG. 20.
As shown, in the optical system in this case, the focus mechanism 125 for the servo light for changing the collimation of the servo laser beam incident to the objective lens 118 and independently adjusting the focusing position of the servo laser beam is added.
In this case, during reproduction, while a recording/reproducing light servo circuit 129 performs focus servo control of the objective lens 118 (the biaxial actuator 119) based on information about the reflected light of the recording/reproducing laser beam obtained by the photodetector 121, a servo circuit 130 for a servo light drives the lens driving unit 126 of the focus mechanism 125 for the servo light based on information about the reflected light of the servo laser beam obtained by the photodetector 128 in the drawing, thus the focus servo control is performed such that the servo laser beam is focused on the reference surface Ref.
Accordingly, it is possible to perform control such that the focusing position of the servo laser beam during reproduction follows the reference surface Ref.
In addition, in FIG. 23, for confirmation, the content of the control performed to the focus mechanisms of the focus mechanism 114 for the recording/reproducing light, the biaxial actuator 119 and the focus mechanism 125 for the servo light is shown. As shown, with respect to the biaxial actuator 119, during recording, the focus servo control and the tracking servo control based on the reflected light of the servo laser beam by the servo circuit 130 for the servo light are performed. In addition, during reproduction, the focus servo control and the tracking servo control based on the reflected light of the recording/reproducing laser beam by the recording/reproducing light servo circuit 129 are performed.
With respect to the focus mechanism 125 for the servo light, only during reproduction, the focus servo control based on the reflected light of the servo laser beam by the servo circuit 130 for the servo light is performed.
In addition, the focus mechanism 114 for the recording/reproducing light is driven according to the value of the offset corresponding to the information recording layer L as a target.